The following U.S. patents are believed to represent the current state of the art: U.S. Pat. Nos. 4,986,275; 5,143,063; 5,143,073; 5,209,221; 5,301,660; 5,431,621; 5,507,790; 5,526,815; 5,884,631; 6,039,048; 6,071,239; 6,113,558; 6,206,873.